1. Technical Field
The invention relates to a directivity detection device of trajectories of drops issuing from a liquid jet.
More particularly, it deals with control of the functioning of a continuous ink jet print head.
The invention detects whether the drops not printed and issuing from a continuous ink jet are effectively or not directed to the recovery gutter of these drops. It likewise determines the charge synchronisation of drops and allows to know the speed of drops issuing from the continuous jet.
The invention likewise relates to an associated electrostatic sensor, print head and printer with continuous ink jets.
2. Prior Art
Continuous ink jet printer heads comprise functional means well known to the person skilled in the art.
FIG. 1 illustrates such a print head according to the prior art. This head essentially comprises the following functional means, described successively in the direction of progression of the jet:                a drop generator 1 containing electrically conductive ink, kept under pressure by an ink circuit, and emitting at least one ink jet 11,        an individual charge electrode 4 for each ink jet,        an assembly constituted by two deflection plates 2, 3 placed on either side of the trajectory of the jet and downstream of the charge electrode 4,        a recovery gutter 20 for collecting the jet ink not used for printing so as to be returned to the ink circuit and thus be recycled.        
The functionality of these different means is described herein below. The ink contained in the drop generator 1 is issued from at least one calibrated nozzle 10 forming at least one ink jet 11. Under the action of a periodic stimulation device placed upstream of the nozzle (not illustrated), constituted for example by a piezo-electric ceramic placed in the ink, the ink jet breaks off at regular time intervals, corresponding to the period of the stimulation signal, at a precise point of the jet downstream of the nozzle. This forced fragmentation of the ink jet is usually caused at a so-called “break-up” point 13 of the jet by periodic vibrations of the stimulation device. At the location of this break-up point the continuous jet transforms into a spatial sequence 11 of evenly spaced identical ink drops. This drop sequence is directed according to a trajectory colinear to the axis of ejection of the jet which theoretically joins the centre of the recovery gutter 20, by geometric construction. Without the effect of external forces, the real trajectory of the drops follows a so-called “static” direction which can be slightly different from the theoretical direction in question, on the one hand because of the imprecisions in manufacture which produce an error of fixed orientation, and on the other hand, due to a drift of the orientation of the jet during operation due to changes in operating conditions of the jet by the nozzle. These changes can be caused in particular by modification of the surface conditions in and around the nozzle caused by accumulation of ink fouling. This problem becomes particularly sensitive after long periods of operation of the printer.
The charge electrode 4, located near the break-up point of the jet, is intended to selectively charge each of the drops formed at a predetermined electric charge value. To do this, with the ink being kept at a fixed electrical potential in the drop generator, a determined electric tension is applied to the charge electrode, different to each drop period. In order for the drop to be charged correctly, the instant of application of the electrical tension must take place slightly prior to instant of break up of the jet so that the electric continuity of the jet is ensured and a given quantity of charges is attracted by electrostatic influence to the tip of the jet. It is therefore necessary to synchronise perfectly the instant of application of the charge tension with the breakup process of the jet.
The two deflection plates 2, 3 are electrically driven to a relative fixed potential of a high value which produces an electrical field Ed substantially perpendicular to the trajectory of the drops. This field can deflect the electrically charged drops which engage between the plates, by an amplitude which is a function of their charge and of the speed of these drops. These deflected trajectories 12 are not collected by the gutter 20 and impact the medium to be printed 30. The placement of the drops on the drop impact matrix to be printed on the substrate is obtained by the combination of individual deflection given to the drops of the jet with relative displacement between the head and the medium to be printed. These two deflection plates 2, 3 are in general flat. One of them can also have an incurved profile or can be arranged at a certain angle. A more elaborate construction is that revealed in application FR 2 821 291 filed by the applicant and illustrated in FIGS. 2A and 2B, which are respectively a frontal view of the print head and a side view according to the direction U of FIG. 2A. In this construction, the two plates are curved and substantially parallel to one another. The plate 2 is concave relative to the median trajectory 15 of the drops while the plate 3 is convex relative to the median trajectory 15. The concave plate 2 is kept at zero potential and is fitted with a slot 16 to let the non-deflected or weakly deflected drops pass. Such an arrangement of plates is highly effective for deflecting drops as the electrical deflecting field remains substantially perpendicular to the trajectories irrespective of the angle of deflection.
The recovery gutter 20 comprises at its inlet an opening 21 whereof the effective section is the projection of its inlet surface onto a plane perpendicular to the nominal axis of the non-deflected jet, placed just upstream in contact with the gutter. In the context of the invention this plane will be called an inlet plane of the gutter. Within the scope of the invention the nominal axis of the non-deflected jet is understood to mean the theoretical axis of the jet when all the sub-assemblies of the head are manufactured and placed relative to one another nominally once the head is assembled. In a print head with curved plates such as described in application FR 2 821 291, the gutter 20 can be positioned more upstream than the lower end of the deflection plates 2, 3 due to the presence of the slot 16, as illustrated in FIG. 2B. This upstream positioning further reduces the flight distance of the drops in the head and thus makes accurate control of the deflection of the drops easier. The performance of the printer, especially the printing quality, is consequently improved by greater placement precision of the drops.
It is known that control of the functioning of a continuous jet print head further requires functional means described earlier, using a certain number of complementary means allowing on the one hand deflection of the drops (which is determined to a large part by the electric charge and the speed of the drops) to be controlled and on the other hand the monitoring of the proper functioning of the recovery of non-printed drops.
With respect to controlling deflection of the drops, it is known to implement dedicated means especially to ensure, on the one hand, synchronised application of the charge signal of the drops with the instant of break-up of the jet (called synchronisation of the charge), and on the other hand, to measure the speed of the drops Vg in order to servo control it them to a preset value. To do this, the print heads according to the prior art generally comprise a measuring device of a representative magnitude of the charge carried by the drops. This measuring device is arranged downstream of the charge electrode. As this charge measuring is carried out, in general, when the specifically charged drops pass in front of this device, the method usually adopted to select the synchronisation instant of the charge relative to the break-up consists of performing repeated trials for changing sequences of drops with a succession of electrical charge signal of shorter duration than the drop period, but with different charge instants (also called “phases”) differently distributed throughout a drop period, and for each assigned phase, to measure the level of charge carried by the drop. The charge level is representative of the efficiency of the charging process of the drops and therefore of the suitability of the charge synchronisation. Some phases produce mediocre or even very poor charge synchronisation, but in general, a certain number of phases permits maximum charge. The charge phase to be used in printing will be selected from the latter. According to the solutions operated for measuring the charge of the drops in view of charge synchronisation, it is generally possible to deduce, in addition to these measurements of charge of the drops, an effective measurement of the speed of the charged drops. In fact, by detecting certain characteristic instants corresponding to the presence of drops identified at different characteristic geometric locations of the print head, it is possible to deduce there from an average travel time of the drops between these known locations, and therefore an average speed of the drops between these locations.
Among all the devices of the prior art, electrostatic sensors are generally used to fulfil this function.
Such a sensor is described for example in patent U.S. Pat. No. 6,357,860 assigned to Linx company and is constituted by two flat electrodes spaced along the trajectory of drops and forming an integral part of one of the deflection plates. This double-electrode sensor provides a signal when charged drops pass in front of each electrode: the amplitude of the signal is representative of the quantity of embedded charge per drop and the time offset between detection by each of the two electrodes give the duration of flight. The speed of the drops of the jet between these two points whereof the separation distance is known can thus be deduced. The advantage of this solution of sensors placed at the level of the deflection plates is to not increase the distance of flight of the drops in the head between the ejection nozzle and the medium to be printed. On the contrary, the disadvantage here is to expose the sensor to significant electrostatic perturbations, especially generated by the noise produced by the circulation of charged drops in the internal environment of the print head and by the noise emitted by the different internal components of the head, which are subjected to variable or noisy electric voltages. These conditions do not allow very precise measurements due to the very noisy signal of the sensor.
Patent EP 0 362 101 B1 in the name of the applicant describes a single electrostatic sensor placed between the charge electrode and the deflection plates, as well as the processing of the associated signal. The sensitive core of this sensor and the circulation space of the charged drops in front of this sensitive core are protected from electrostatic perturbations by electrostatic shielding. The presence of specifically charged drops is detected by their electrostatic influence on the sensitive core of the sensor. The exploitation of the signal obtained from these drops passing in front of this sensor takes very precise measurements of the charge level of these drops and defines the instants of their entry to and exit from the sensor, therefore the transit duration of these drops in the detection zone of the sensor. If the effective length of the zone travelled through is known, the average speed of the drops passing in front of the sensor can be deduced.
With respect to monitoring of the collection of non-printed drops, it is known to use dedicated means to detect that the ink not used for printing is properly recovered. If this ink escapes the gutter, the jet must be stopped to avoid fouling of the print head and its environment, fouling being generally unacceptable to the user of the printer. These problems can be created by deficiency of the recovery device which is incapable of evacuating the ink of the non-printed drops or by abnormal behaviour of the jet. In fact, the orientation of the jet can vary, such as for example be set at start-up at a value different to the nominal value or can move away from the nominal value during operation. No functional problem occurs as trajectories of the drops not intended for printing reach the interior of the gutter. On the contrary, dysfunctioning appears when the trajectory of the jet exits from the gutter or when drops strike its edge. Recovery detection can be done in different ways, especially by analysis of the resistivity of the fluid vein of the return circuit of the ink immediately downstream of the inlet of the gutter. Unfortunately, the system can be faulty since it cannot generally make the difference between the case of correct functioning and that where the jet, when improperly oriented, strikes the edge of the gutter. In this case, part of the ink enters the gutter to create the conditions which the resistivity sensor will interpret as a jet partially recovered by the gutter, a situation also characteristic of normal printing. So, in a situation where the jet is improperly oriented all or part of the ink of the jet contaminates the immediate environment of the edge of the gutter, or flows inside the gutter, which generally results in major dysfunction after it accumulates. The detection of correct recovery of the ink inside the gutter is therefore not reliable with solutions of the prior art.
This is why a certain number of solutions using sensors for locating the drops in this case has already been proposed. The localisation of ink drops by physical contact on a pressure sensor or by means of optical barriers is not reliable under industrial conditions of use of ink jet printers, due in particular to the sensitivity of such solutions to fouling by ink.
Other solutions according to the prior art consist in using electrostatic sensors, in so far as the liquid which makes the drops have come is conductive, the latter able to be charged electrically. The general principle uses the property according to which the level of the signal detected by an electrostatic sensor, during the passage of electric charges, depends on the distance between the active surface of the sensor and the charged drops. The localisation principle of the charged drops according to the state of the art consists in using two electrostatic sensors placed symmetrically on either side of the trajectory of drops the spacing of which relative to a nominal trajectory is to be evaluated. The difference in amplitude of the current signals delivered when charged drops pass in front of the sensors indicates the real position of the drops relative to the sensors in a certain single direction.
U.S. Pat. No. 3,886,564 assigned to IBM company describes several types of arrangement of pairs of electrostatic sensors, delivering signals whereof the differential processing determines the relative position of the drops passing in front of the sensors. The detection of position of charged drops in two directions defining a plane cutting the trajectory of these drops requires an arrangement of four electrostatic sensors arranged in two pairs and the implementation of electronics and the associated signal processing.
U.S. Pat. No. 4,551,731 and EP 0 036 789 assigned to Cambridge Consultants company describe this type of arrangement definitively requiring four sensors per trajectory of drops to be monitored for evaluating the drift, in two directions of the real trajectory of the drops relative to a nominal trajectory in passing in front of the sensors. Using this principle on a continuous ink jet print head leads to complex, bulky and costly implementation. This realisation causes other disadvantages:                on the one hand, the use of four sensors placed around the jet cannot be done without partially masking visibility the jet which is confined at the level of the sensors in a narrow space, difficult to access for maintenance of the print head, especially for cleaning the charge or deflection elements;        on the other hand, the means which are dedicated to measuring the orientation drift of the jet must be inserted along the trajectory of the jet between the nozzle and the recovery gutter. The intrinsic bulkiness of the sensors generates problems of physical integration and tends to increase the distance of flight of the drops between their charge and their impact locations on the medium to be printed. The drawback is that a long distance of flight of drops impairs position precision of impacts and therefore the printing quality.        
In summary, the major disadvantages of recovery detection solutions of drop coming from liquid jet according to the prior art are the following:                detection of the passage of the ink in the gutter by means of a sensor analysing the ink flow in the fluid vein in the gutter is not enough to prevent pollution risks because when the jet strikes the edge of the gutter it is not detected as a defect situation,        evaluation of the real position of the drops, at the level of a plane perpendicular to the nominal trajectory of the jet and in the vicinity of the inlet of the gutter, is possible with solutions of the art using several pairs of electrostatic sensors but at the price of significant bulkiness and at prohibitive cost;        arrangement of two pairs of electrostatic sensors around the jet makes it very difficult to access the different functional means of the head for maintenance, especially for cleaning;        using sensors dedicated to measuring orientation shifts of the jet on the trajectory of the jet makes the drop flight paths longer in the print head to the detriment of the print quality;        using electrostatic sensors easily perturbated by noise coming from different electric signals of the print head and from electric charges in movement in the print head affects measurement precision. It is frequently necessary to either create effective shielding, often in a bulky manner, of the sensitive parts of the sensor, or to perform additional processing of the signal produced, which proves costly.        
The aim of the invention is therefore to eliminate the drawbacks of the prior art.
A particular aim of the invention is to propose a reliable and inexpensive solution for detection of the directivity of trajectories of ink drops issuing from a continuous jet in a print head, which ensures rapid detection of operating defects and optimal management of these possible defects to limit the harmful consequences for the user of the printer equipped with the head.